The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio at a transient state.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
When starting the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced at a speed which is determined by line pressure, the pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio. In such a system, the speed of changing of transmission ratio up to a desired transmission ratio can not be controlled in accordance with driving conditions. Accordingly, hunting or overshooting of the transmission ratio occurs, which decrease the driveability of the vehicle.
EP-A-207603 discloses a system employing a desired transmission ratio lookup table from which a desired transmission ratio id is derived in accordance with the opening degree .theta. of a throttle valve and the driven pulley speed N.sub.S or vehicle speed. The system operates to control the transmission ratio to the desired transmission ratio.
However, since the values of desired transmission ratio stored in the lookup table are provided for steady state, the data derived from the lookup table can not be used for transient state. Therefore, another means for setting desired transmission ratio besides the above described lookup table for steady state is necessary for the transient state. Thus, the control system becomes complicated.
In order to solve such a problem, the inventor of the present invention has proposed a transmission ratio control system which is provided with a single lookup table and may control the transmission ratio by the single table at the transient state also. In the system, a desired drive pulley speed N.sub.P d is obtained from a look-up table dependent on engine load .theta. and actual transmission ratio i, and desired transmission id is derived from a look-up table dependent on the derived speed N.sub.P d and driven pulley speed N.sub.S. Thus, transmission ratio changing speed (rate) di/dt is calculated from the difference between the actual transmission ratio i and the desired transmissions ratio id and desired transmission ratio changing speed (rate) did/dt. The calculation of transmission ratio changing speed di/dt is as follows. EQU di/dt=K1(id-i)+K2.multidot.did/dt
where K1 and K2 are coefficients.
In accordance with the system, transmission ratio can be controlled in various driving conditions including transient state.
However, since the coefficient K2 is constant, the system cannot properly control the transmission ratio in accordance with various driving conditions, as described below. Referring to FIG. 4, when the kickdown is operated at points A and C, the transmission is temporarily downshifted. At the kickdown at the point A where the engine speed Ne and the vehicle speed V are low, the transmission is downshifted from a minimum transmission ratio line Imin to a larger transmission ratio D1 on a larger transmission ratio line IB which is determined by the engine speed and the vehicle speed. At the kickdown at the point C where the engine speed and the vehicle speed are higher than point A, the transmission is downshifted from the minimum transmission ratio line Imin to a transmission ratio D2 on a smaller transmission line IC than line IB. Thus, the difference IC-Imin is smaller than IB-Imin. When the coefficient K2 is set at a small value to provide a slow downshifting speed so as to avoid shock at larger downshift at point A, such a downshifting speed is too slow for the downshifting at point C, which causes reduction of driveability of the vehicle in active driving.